Pilot signals are a vital component in any communication system. The pilot signals are used at the receiver to estimate the channel, to provide a phase reference for the demodulation and to estimate the received signal for link adaptation and/or power control. There are two types of pilot signals (or pilots): dedicated or common. A dedicated pilot can be used by one receiver only, whereas a common pilot can be used by several receivers. A common pilot signal only makes sense in a point-to-multipoint channel.
A Base Station Controller, BSC, or a Radio Network Controller, RNC, determines which pilot signal(s) a mobile station shall use. The BSC or RNC may base the decision of which pilot channel(s) to use on several different parameters or factors. Hereinafter, when referring to an RNC, also a BSC may be intended. The RNC will receive measurement reports from the mobile stations via base stations. Further, the base stations may perform measurements regarding e.g. signal quality and report these measurements to the RNC.
In certain circumstances, a mobile station may need to switch between different pilot signals within a cell. There may be several reasons for frequent changes of pilot channels. For example, the mobile station may be moving around within the cell such that it is necessary to switch pilot channels. According to another example, the radio conditions at the particular location of the mobile station may be such that switching of pilot channels will occur frequently.
In such cases, large amount of resources are required of the RNC to constantly perform calculations in order to optimize the use of the available pilot channels within the cell. Further, in such cases, the interface between the RNC and the base station will be heavily loaded with different kinds of signaling.
In WCDMA, common pilots are commonly used in the downlink (DL). In the 3GPP standard, two types of DL common pilots have been defined. They are called Primary Common Pilot Channel (P-CPICH) and Secondary Common Pilot Channel (S-CPICH). The P-CPICH uses a unique (defined in the standard) channelization code, and there can be only one P-CPICH in each cell.
The default is that the mobile station uses the P-CPICH for all relevant purposes. However, under some circumstances, the S-CPICH is preferred. When a mobile station detects that one or several S-CPICH are transmitted in its vicinity, it transmits a measurement report to the radio network controller (RNC), indicating which S-CPICH it has detected and the relative merits of this S-CPICH.
Upon reception of such a measurement report, the RNC may determine that the mobile station should start using the S-CPICH, rather than the default P-CPICH. The RNC then sends a message to the mobile station via the base station. When the mobile station receives this message, it acknowledges the message and starts using the S-CPICH rather than the P-CPICH.
The procedure is the same when the mobile station is currently using one S-CPICH and desires to start using another S-CPICH.
Originally, the S-CPICH was introduced to enable the application of adaptive antennas, where one antenna with multiple beams covers one cell, and any dedicated data transmission takes place over one of the beams, thereby reducing the interference in the system. The area covered by one beam is called a cell portion. Different S-CPICHs must be used in different beams. Note that in the UL, the base station would receive signals from all beams, and use them all to decode the signal transmitted from a mobile.
What beam is used for transmission is decided by the Radio Network Controller (RNC). This decision is primarily based on measurement reports from the mobile, but measurements from the base station may also be taken into account.
When the decision has been made, the RNC then orders the base station to start transmitting over the selected beam. As previously described, the RNC also signals to the mobile to start using a new S-CPICH. With these actions, the RNC implicitly orders the mobile station to receive the data transmitted over the new beam. The RNC uses Radio Resource Control (RRC) signaling to convey the S-CPICH command to the mobile.
Another application of the S-CPICH is a system using remote radio units (RRUs). Here, the radio signals are transmitted from remote radio head (RHs). Each radio head is connected to a central node using optical fibers. Traditionally, each RH creates an entire cell, but an alternative application would be that each RH creates a cell portion. Mobility between cell portions is handled as for the multi-beam case described above.
Yet another application of the S-CPICH is dynamic, mobile-specific beam forming. Here, each base station antenna is equipped with several antenna elements, where the phase and amplitude of the signal transmitted from each antenna element can be dynamically controlled. By performing individual adjustment of the phase and amplitude of each antenna element, it becomes possible to steer the transmission with a high degree of freedom. Not only is it possible to transmit the signal in certain directions, it is also possible to avoid transmitting interference in other directions. To facilitate demodulation of the signal at the receiver, a pilot signal has to be transmitted using the same antenna element weights. Here, the P-CPICH cannot be used. Instead, the S-CPICH may be used.
There are, however, some problems associated with the known solution. The problems become even more palpable when a mobile station is close to a border between cell portions and there is a need to frequently assign a new S-CPICH. The process is slow due to filtering of the measurements in the mobile and the signaling delay. The filtered measurement needs to be above a certain level for a certain time before it is transmitted from the terminal. The processing capacity of the RNC limits the rate with which the pilot assignment can be updated and the use of downlink measurements makes it necessary for the S-CPICH to be transmitted continuously, which wastes capacity.